const arr = ['a','b', 'c']
let v
function defineReactive (data, key, val) {
    Object.defineProperty(data, key, {
        get: function(){
            console.log('get:', key)
            return key
        },
        set:function(newVal){
            console.log('data:', newVal, val)
            val = newVal
            update
        }
    })
}

function observe(data) {
    console.log(Object.keys(data))
    Object.keys(data).forEach((k) => {
        defineReactive(data, k, data[k])
    })
}

defineReactive(arr, '1')



